Help ! One of my students accidentally melted 10 g of silver into
approximately 19 g of 9ct gold, consequently the resulting alloy is
the colour of light brass. Has anyone any ideas on how to separate
the gold? We will consider any process that involves disolving the
copper/silver with acids.Regards, Brian Minnear

Help ! One of my students accidentally melted 10 g of silver
into approximately 19 g of 9ct gold, consequently the resulting
alloy is the colour of light brass. Has anyone any ideas on how to
separate the gold? We will consider any process that involves
disolving the copper/silver with acids.Regards, Brian Minnear

Add enough copper or silver to the mix so the gold content is less
than about 25%. Then roll thin, and put in 50% nitric acid. The
other metals will dissolve, leaving the pure gold.

The gold can be separated from your new unwanted alloy by digestion
in dilute nitric acid. Dilute nitric acid acts on 6K or less gold
alloys to dissolve silver, copper, etc. leaving gold as a fine brown
to blackish powder (assuming no platinum group metals are present).
The procedure is straightforward but can be hazardous if proper
procedures are not followed.

The addition of 10 g. of silver (sterling?) to the 9K alloy reduced
the quality to 5.9K. This assumes there was 19.0 g. of the 9K
alloy. If there was much more of the 9K alloy than 19 g., it will
be necessary to add more silver or copper. The metal is converted
to grain by melting and pouring slowly into water. The grain is
reacted with a 1:1 by volume conc. nitric acid to distilled water…
The nitric acid should be at full strength. Shelf life of
laboratory grade conc. nitric acid is one year. The acid is added
in small portions. More is added when the previous portion is spent
until the digestion is complete. The high purity gold (22 - 23K)
remaining is filtered off after dilution and then melted. The
silver can be recovered from the spent acid solution if desired.

I can provide you with details of the procedure by e-mail. It is
important to note that nitric acid is very corrosive and is a
powerful oxidizing agent. A very toxic gas (nitrogen dioxide) is
given off during the digestion. The process is not difficult but
must be carried out using proper laboratory procedures.

Brian, silver and copper, but not the gold, will be dissolved by a
solution of ferrous nitrate, or by nitric acid. The action is
hastened by heating. Make the operator aware of the safety
considerations, especially if using the nitric acid (you’ll have to
research this yourself, but one of the concerns would be the
evolution of nitrogen dioxide gas). If you are at a school, as you
seem to be, it might be best to get a technician to do the job,
particularly as it won’t be quick.

The liquid only works where it touches, and I cannot say whether you
would be able to extract all the solver and copper from a lump of
alloy and be left with gold sponge, or not. Might just work in this
case, with a relatively low concentration of gold. What you could
try would be to extract for a while, then melt the residue and pour
from a height into cold water to make granules, and put these back
into the acid and extract again, and again…

You could also research the cupellation technique, which is one
method for determining the fineness of gold by removing all base
metals and silver, but you need a high temperature kiln and the right
type of porous crucible, so acid may be the best way to go.